Features of the EAP events on the medium-range evolution process and the mid-and high-latitude Rossby wave activities during the Meiyu period

In this paper,features for the evolution of the East Asia/Pacific(EAP) events and their association with high-and mid-latitude Rossby waves during the Meiyu period are analyzed on the medium-range time scale.It is shown that life cycles of the positive and negative EAP events cannot be simply regarded as "mirror" each other.In the upper troposphere,downward propagations of Rossby wave packets both over high-and mid-latitude regions of Eurasian continent and over the Asian jet region are responsible for generating basic patterns of high-and mid-latitude anomaly centers of the events.In this layer,Rossby wave packets also propagate from the mid-latitude anomaly center to the high-latitude one.In the middle and lower troposphere,the formation of the subtropical anomaly center of the event is mainly attributed to the anomalous convective activity in the tropical Pacific warm pool.The northward Rossby wave energy dispersion from this center is favorable to the enhancement and maintenance of the mid-latitude anomaly center in the same layer.Finally,it might be hypothesized that typical features of the positive and negative EAP events in their mature phase result from the interaction between(or phase-locking of) respective anomalous circulations induced both by quasi-zonal Rossby wave packets embedded in upper troposphere westerly and by quasi-meridional Rossby wave packets in the background flow of the East Asian summer monsoon in the middle and lower troposphere.

The onset and advance of the Asian summer monsoon

Based on the daily outgoing longwave radiation (OLR) data from National Oceanic and At- mospheric Administration (NOAA) satellites, the Cli- mate Prediction Center’s merged analysis of pre- cipitation (CMAP) data and the National Centers for Environmental Prediction/National Center for At- mospheric Research (NCEP/NCAR) reanalysis dataset, the mean intraseasonal variability of the Asian summer monsoon (ASM) is investigated by using power spectrum analysis, band-pass filter, and diagnostic analyses. The processes of the onset and advance of monsoon over the southern part of Indo- china Peninsula, the east coast of Bay of Bengal, the South China Sea and the Indian subcontinent are explored. It is found that there is an abrupt change in OLR, precipitation and zonal wind during the onset and advance of the ASM. It is also indicated that the southern part of Indochina Peninsula and the adja- cent Andaman Sea is the region where the earliest onset of the ASM occurs in the 2nd pentad of May.

PREFACE

Since the first evidence of organized large-amplitude intraseasonal oscillations was report-ed by Madden and Julian in the early 1970s, it has been well known to us that there is anatmospheric variability on time scales of about 3O-60 days (centered around 40-60 days).These oscillations are particularly evident in rainfall and wind fluctuations over the IndianOcean and the Indonesian-north Australian sectors of the tropics. It Is only very recentlythat 30-60 day oscillations have been found (observationally) to exist all over the globe,especially in both tropics and middle latitudes.